This invention relates to inhibiting the corrosion of stainless steel that occurs in aqueous solutions of alkali metal hydroxides at temperatures of less than 100.degree. C. In particular, it relates to the use of about 2 ppm to about 500 ppm wt % of sodium borohydride, hydrazine, or a mixture thereof, to inhibit corrosion of the stainless steel at that temperature.
Caustic soda (NaOH) is manufactured commercially by the electrolysis of brine. The materials of construction in that process for caustic storage and recirculation are often made of stainless steel. While stainless steel is a relatively inert material in that environment, it is nevertheless subject to gradual electrochemical corrosion. As the stainless steel corrodes, iron enters the solution, which not only lowers the product quality of the caustic being produced and but also reduces the efficiency of the catalytic cathodes by depositing iron on them and increasing the cell voltage. Iron, as HFeO.sub.2.sup.31 , also migrates through the cell membrane and deposits on the anode as an iron oxide, which increases the anode overvoltage, especially when the noble metal loading on the anode is low. In addition, the corroding materials must be periodically removed and cleaned with an acid wash.
U.S. Pat. No. 4,585,579 discloses the use of sodium borohydride to inhibit the corrosion of nickel in caustic solutions at temperatures of about 100.degree. to about 200.degree. C. However, until now, a good corrosion inhibitor for stainless steel in caustic solutions at temperatures below 100.degree. C. has not been identified.